The present invention relates to an apparatus and method for sorting foreign bodies from prime material, and more particularly to an apparatus and method for sorting foreign bodies from such prime material as raw tobacco or tobacco ribs wherein prime material on a moving conveyor belt is irradiated with visible and/or invisible light, the reflected light being picked up by an evaluating device in spatial or time sequence in the form of line areas moving at right angles to the direction of movement, said evaluating device integrating over time the signals obtained from several line areas and providing operating signals to an ejection device, the ejection device removing the detected foreign body from the prime material.
In a known method and apparatus as disclosed in U.S. Pat. No. 3,097,744 particles are removed from a flow of like particles, as contrasted with the removal of foreign bodies from a flow of different material. In particular, the entire width of the region traversed by the particles under investigation is scanned line by line. The individual signals obtained by optically scanning line by line are integrated to obtain a signal which, on rising above or dropping below a threshold value characteristic of the particles, brings about a discharge of unwanted particles from the particle flow.
Such an apparatus and method fail to function satisfactorily under certain conditions, for example particularly if the region under investigation is relatively wide or if a relatively large number of juxtaposed particles passes through this region. Unsatisfactory functioning results because only the small scanning area taken up by a particle to be removed contributes to the formation of an ejection signal, while the remaining, larger part of the overall scanning width contributes a signal which merely fluctuates to a greater or lesser extent about a mean value as a function of the noise. Even if the signals obtained in this way by line by line scanning are upslope integrated, in the presence of a particle to be ejected the total value obtained varies very slightly compared with a state in which no particle to be ejected is detected. This means that the method and apparatus are relatively insensitive or that extremely high quality signal evaluating devices must be used.
In another known apparatus disclosed in German OS 2,015,108, which operates in accordance with the same principle as the known apparatus described hereinbefore, individual mineral fragments successively drop past a camera tube, which records the reflections resulting from the illumination of the fragments and feeds them line by line to threshold circuits. The number of signals supplied by the threshold circuits is then a measure of the reflection behaviour of the mineral fragment surface and can be used for sorting particular mineral fragments from the flow of individual, successively following mineral fragments.
Thus, in this apparatus, only individual successively following particles are investigated and possibly removed and for the reasons indicated hereinbefore there is a considerable reduction in the sensitivity of this apparatus on changing to a larger number of juxtaposed particles or to a larger area scanning region.
Furthermore, a method and an apparatus are known, as disclosed in German Pat. No. 1,946,615, by means of which foreign bodies such as cigarette paper and lining paper fragments, filters and the like can be removed from pulled apart cigarette waste. To this end, groups of photocells are arranged above a conveyor belt on which the cigarette waste is conveyed and at right angles to the direction of movement thereof. Each of said photocells is allocated to a portion of the belt so that the entire belt width is scanned. The conveyor belt and the materials conveyed on it are illuminated in the vicinity of these photocell groups. The incident light is passed through color filters in order to block light having frequencies reflected by tobacco fibers (the prime material) and the conveyor belt, but admit light that is strongly reflected by the generally light foreign bodies to the corresponding photocells. The signals produced by illuminating the photocells are then supplied to electromagnets to activate ejection devices. The ejection devices are in the form of suction mechanisms covered by flaps, and are associated with the respective portions of the belt having an allocated photocell so that the detected foreign body or bodies can be removed.
This known method and apparatus, however, are suitable only for uses in which the foreign bodies have a much greater reflectivity than the prime material, particularly due to the fact that in this method the signals are obtained by evaluating the total brightness of a relatively large area of the conveyor belt and the materials located in this area.